[Illustration:  Fig. 37.—­Astylospongia proemorsa, cut vertically so as to exhibit the canal-system in the interior.  Lower Silurian, Tennessee. (After Ferdinand Roemer.)]

The most abundant, and at the same time the least understood, of Lower Silurian Protozoans belong, however, to the genera Stromatopora and Receptaculites, the structure of which can merely be alluded to here.  The specimens of Stromatopora (fig. 38) occur as hemispherical, pear-shaped, globular, or irregular masses, often of very considerable size, and sometimes demonstrably attached to foreign bodies.  In their structure these masses consist of numerous thin calcareous laminae, usually arranged concentrically, and separated by narrow interspaces.  These interspaces are generally crossed by numerous vertical calcareous pillars, giving the vertical section of the fossil a lattice-like appearance.  There are also usually minute pores in the concentric laminae, by which the successive interspaces are placed in communication; and sometimes the surface presents large rounded openings, which appear to correspond with the water-canals of the Sponges.  Upon the whole, though presenting some curious affinities to the calcareous Sponges, Stromatopora is perhaps more properly regarded as a gigantic Foraminifer.  If this view be correct, it is of special interest as being probably the nearest ally of Eozooen, the general appearance of the two being strikingly similar, though their minute structure is not at all the same.  Lastly, in the fossils known as Receptaculites and Ischadites we are also presented with certain singular Lower Silurian Protozoans, which may with great probability be regarded as gigantic Foraminifera.  Their structure is very complex; but fragments are easily recognised by the fact that the exterior is covered with numerous rhomboidal calcareous plates, closely fitting together, and arranged in peculiar intersecting curves, presenting very much the appearance of the engine-turned case of a watch.

[Illustration:  Fig. 38.—­A small and perfect specimen of Stromatopora rugosa, of the natural size, from the Trenton Limestone of Canada. (After Billings.)]

Passing next to the sub-kingdom of Coelenterate animals (Zoophytes, Corals, &c.), we find that this great group, almost or wholly absent in the Cambrian, is represented in Lower Silurian deposits by a great number of forms belonging on the one hand to the true Corals, and en the other hand to the singular family of the Graptolites.  If we except certain plant-like fossils which probably belong rather to the Sertularians or the Polyzoans (e.g., Dictyonema, Dendrograptus, &c.), the family of the Graptolites may be regarded as exclusively Silurian in its distribution.  Not only is this the case, but it attained its maximum development almost upon its first appearance, in the Arenig Rocks; and whilst represented by a great variety